Microvalve for controlling fluid flow

ABSTRACT

A microvalve for controlling fluid flow, including a body portion having a plurality of spaced openings formed therein, a shutter located adjacent to and substantially parallel with the body portion having a plurality of spaced openings formed therein, a drive mechanism for causing the shutter to move laterally with respect to the body portion so that the spaced openings of the shutter are brought into and out of alignment with the spaced openings of the body portion, wherein the microvalve is in an open position and a closed position, respectively, and, a latching mechanism for preventing the shutter from moving laterally with respect to the body portion.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of Ser. No. 10/048,082, filed Jan. 24,2002, which claims priority to and is a 371 of PCT/US 00/19785 filedJul. 24, 2002, which claims the benefit of 60/175,152, filed Jan. 7,2000, and the benefit of 60/146,625, filed Jul. 30, 1999.

FIELD OF THE INVENTION

This invention relates to a microvalve for controlling fluid flow, andmore particularly, to a latching system utilized with such microvalve.

BACKGROUND OF THE INVENTION

Microvalves employed to control the flow of fluid are presently in use,with several designs falling within a class known as microelectromechanical systems or “MEMS.” It will be appreciated that suchmicrovalves are preferably driven thermally or electrostatically. Ineither case, slots or other types of openings are placed in an open orclosed position, respectively, preferably within a shutter-typeconfiguration so as to permit or prevent fluid from flowingtherethrough. Accordingly, the greatest amount of opening can beaccomplished by a minimal amount of movement.

It has been determined, however, that the prior art microvalves have anundesirable amount of power drain associated therewith. This is becausethe power required to open the microvalve must be maintained in order tokeep it in the open position. By latching the movable portion of amicrovalve shutter in the open position, the need for continuous powerthereto would be eliminated. One example of latching is disclosed inU.S. Pat. No. 5,837,394 to Schumm, Jr., where a detent or ratchet isprovided to assist in holding a sliding portion of a semiconductormicroactuator in either the open or closed position. As seen therein,though, separate actuators are utilized to move the sliding portion ineach direction. In this way, the actuators must overcome the resistanceprovided by the detent/ratchet so that the sliding portion is able tomove into the desired position. This clearly requires a greater forcefrom the actuators, and therefore a greater amount of power to theactuators. Further, it will be seen that the '394 patent relatesspecifically to electrically activated, thermally responsivesemiconductor valves that include and contain a cantilever deformableelement which deforms on heating by electrical resistance.

It will further be appreciated that while microvalves of the typedisclosed herein may be utilized in any number of environments, onespecific application has been in the field of metal-air batteries.Metal-air batteries have decided advantages over other types ofelectrochemical cells such as typical alkaline (zinc/manganese dioxide)or lithium batteries. The metal-air batteries utilize a gas reactant,such as oxygen or air, which does not have to be stored in the batterylike a solid reactant. The gas reactant may enter the cell through ventsor holes in the battery case. Thus, metal-air batteries are able toprovide a higher energy density (watts per unit mass) that may result ina relatively higher power output and a relatively lower weight. This isparticularly useful in applications in which a small, light battery isdesired so that more energy is provided in the same size package or thesame amount of energy in a smaller package. Metal-air batteries are alsoenvironmentally safe and generally leakage-free.

Metal-air batteries are comprised of one or more electrochemical cells.Each cell typically includes a metal anode and an air cathode with aseparator electrically isolating the two, where an electrolyte ispresent, in the anode, cathode and separator. The metal anode usuallycomprises a fine-grained metal powder, such as, but not limited to,zinc, aluminum or magnesium, blended together with an aqueouselectrolyte, such as potassium hydroxide, and a gelling agent into apaste. The air cathode is a catalytic structure designed to facilitatethe reduction of oxygen and typically comprises active carbon, a binderand a catalyst, which are formed into a sheet together with a metalcurrent collector. The air cathode also commonly incorporates ahydrophobic polymer, such as polytetrafluoroethylene or polypropylene,directly into the cathode sheet and/or as a coextensive film. Thehydrophobic polymer prevents the electrolyte from passing through thecathode and leaking from the cell.

In a metal-air battery, oxygen, through a series of reactions, reactswith the metal in the cell producing electrical current. In a zinc-aircell, for example, oxygen enables a charge/discharge reaction at thecathode (positive electrode):½O₂+H₂O+2e⁻⇄2OH⁻.

Meanwhile, a charge/discharge reaction occurs at the anode (negativeelectrode):Zn+2OH⁻⇄ZnO+H₂O+2e⁻.

Hence, the zinc-air cell has an overall reaction:Zn+½O₂⇄ZnO.

Typically, metal-air batteries utilize ambient air, which containsapproximately 21% oxygen, as the reactant for the cells. The ambient airenters through ventilation holes in the housing. In the housing, theoxygen in the ambient air reacts with the cells. The oxygen-depleted airthen exits the housing. Thus, ambient air enters or is drawn into thehousing in a flow sufficient to achieve the desired power output.

Free flow of ambient air through the metal-air cell, however, createsseveral problems that may lower the efficiency of a metal-air cell oreven cause the cell to fail prematurely. First, ambient air that entersthe electrochemical cell will continue to react with the anoderegardless of whether the cell is providing electrical energy to a load.Thus, the capacity of the cell will continue to decrease unless air isexcluded while the cell is not providing electrical energy to a load.Another problem with allowing free flow of ambient air as the reactantis the difficulty in maintaining the proper humidity in the battery.Equilibrium vapor pressure of the metal-air battery results in anequilibrium relative humidity that is typically about 50-60%. If theambient humidity is greater than the equilibrium relative humidity valuefor the metal-air battery, the metal-air battery will absorb water fromthe air through the cathode and fail due to a condition called flooding,which may also cause the battery to leak. If the ambient humidity isless than the equilibrium relative humidity value for the metal-airbattery, the metal-air cells will release water vapor from theelectrolyte through the air cathode and fail due to drying out. Further;impurities such as carbon dioxide (CO₂) present in the ambient air maydecrease the energy capacity of the cell. Thus, a metal-air cell willoperate more efficiently and longer if the flow of ambient air iscontrolled so that the air enters the cell only when the cell isproviding electrical energy to a load.

Air exchange control systems for metal-air batteries have been designedto control the flow of ambient air into and out of metal-air cells forthe following reasons: (1) to prevent the cell from continuing to react;(2) to prevent changes in the cell humidity; and, (3) to prevent CO₂from entering the cell when the battery is not providing electricalenergy to a load. Some designs, for example, use a mechanism physicallyoperated by the user where a valve or vent cover is attached to a switchthat turns an electrical device “on” so that when the switch moves, thecover moves. See, e.g., U.S. Pat. No. 2,468,430, issued to Derksen onApr. 26, 1949; U.S. Pat. No. 4,913,983 entitled “Metal-Air Battery PowerSupply” and issued to Cheiky on Apr. 3, 1990; and, H. R. Espig & D. F.Porter, Power Sources 4: Research and Development in Non-MechanicalElectrical Power Sources, Proceedings of the 8^(th) InternationalSymposium held at Brighton, September 1972 (Oriel Press) at p. 342. Inthese designs, however, the air exchange system requires the physicalpresence of the operator and an electrical device that has a switchcompatible with the battery air exchange system.

Automatic air exchange systems that are contained within the battery andoperate without the presence of a user, however, typically providesignificant parasitic drains on the energy capacity of the cell that mayalso shorten the life of the cell. One design, such as the one disclosedin U.S. Pat. No. 4,177,327 entitled “Metal-Air Battery HavingElectrically Operated Air Access Vent Cover” and issued to Mathews etal. on Dec. 4, 1979, utilizes a vent cover associated with anelectrically operated bimetallic actuator to close the air access ventswhen the battery was not in use to prevent ambient air from entering thehousing when the battery is not in use. This is accomplished by applyinga current to the bimetallic actuator so that the two materials thereofheat up, whereby the different thermal expansion coefficients thereofcause the system to bend up or down. The electrical actuator, however,provides a substantial parasitic drain on the metal-air cells anddiminishes the life of the cell.

Additionally, U.S. Pat. No. 5,304,431 entitled “Fluid DepolarizedElectrochemical Battery with Automatic Valve” and issued to BrookeSchumm, Jr. on Apr. 19, 1994; U.S. Pat. No. 5,449,569 entitled FluidDepolarized Battery with Improved Automatic Valve” and issued to BrookeSchumm, Jr. on Sep. 12, 1995; and U.S. Pat. No. 5,541,016 entitled“Electrical Appliance with Automatic Valve Especially for FluidDepolarized Electrochemical Battery” and issued to Brooke Schumm, Jr. onJul. 30, 1996, disclose a design incorporating a thermally responsivesemiconductor microactuator disposed over a fluid entrance inlet topermit ambient air to enter the cell when the battery is supplyingelectrical power to a load. In this design, electrical energy on theorder of milliwatts is dissipated to heat a resistive element that opensa thermally responsive valve and keeps that valve open while the batteryis in use. Thus, as described hereinabove with respect to the '394patent, the design also provides a continuous parasitic drain on thecell that decreases the life of the cell.

Therefore, there exists a need for a microvalve, particularly oneutilized as an air exchange system in a metal-air battery, thatminimizes the parasitic drain on the cell. There also exists a need tominimize the size of microvalves used with a metal-air battery so thatit fits within a standard battery package and maximizes the volume ofthe battery that is available for providing electrical energy. It isalso desirable that such microvalves be mass produced to decrease costs,as well as enable large numbers of batteries to be assembled containingthem as an air exchange system.

SUMMARY OF THE INVENTION

In a first embodiment of the present invention, a microvalve forcontrolling fluid flow is disclosed as including: a body portion havinga plurality of spaced openings formed therein; a shutter locatedadjacent to and substantially parallel with the body portion, theshutter having a plurality of spaced openings formed therein; a drivemechanism for causing the shutter to move laterally with respect to thebody portion so that the spaced openings of the shutter are brought intoand out of alignment with the spaced openings of the body portion,wherein the microvalve is in an open position and a closed position,respectively; and, a latching mechanism for preventing the shutter frommoving laterally with respect to the body portion.

In a second embodiment of the present invention, a method ofelectrostatically actuating a microvalve between a first position and asecond position is disclosed, wherein the microvalve includes a shutterlocated adjacent a body portion, comprising the following steps:disengaging a latching mechanism so as to permit movement of the shutterwith respect to the body portion; actuating a drive mechanism to movethe shutter from the first position to the second position with respectto the body portion; and, engaging the latching mechanism so as toprevent movement of the shutter from the second position.

In a third embodiment of the present invention, a fluid-breathingvoltaic battery is disclosed as including a container, a voltaic celldisposed within the container, and a fluid exchange system. The fluidexchange system further includes a microvalve having a first state and asecond state, wherein the microvalve is disposed in the container suchthat the microvalve is adapted to allow a fluid into the cell when themicrovalve is in the first state and to substantially prevent the fluidfrom flowing into the cell when the microvalve is in the second state,and a controller electrically connected to the microvalve, wherein thecontroller is adapted to initiate a change of state in the microvalve.

In a fourth embodiment of the present invention, a method of fabricatingan electrostatic microvalve is disclosed as including the followingsteps: providing a first wafer having a top surface and a bottomsurface; providing a masking material on the top surface of the firstwafer; providing a second wafer having a top surface and a bottomsurface; etching a plurality of spaced openings on the top surface ofsaid second wafer; bonding the bottom surface of the first wafer to thetop surface of the second wafer via a sacrificial layer; etching themasking material of the first wafer to create a shutter and a pluralityof actuators operative therewith; etching a portion of the second waferso as to create a passage in flow communication with the spaced openingsetched on the top surface thereof; and, removing a portion of thesacrificial layer between the first and second wafers to release theactuators.

In a fifth embodiment of the present invention, an electrostaticmicrovalve is disclosed as including: a first wafer having a maskingmaterial on a top surface thereof, wherein the top surface is etched tocreate a shutter, a plurality of actuators operative with the shutter,and a latching mechanism to prevent movement of the shutter; a secondwafer having a plurality of spaced openings etched on a top surfacethereof, wherein a portion of the second wafer in substantial alignmentwith the spaced openings is etched therefrom so as to create a passagein flow communication therewith; and, a sacrificial layer positionedbetween the first and second wafers to bond the first and second wafers,the sacrificial layer having a portion removed in substantial alignmentwith the flow passage so as to release the actuators.

BRIEF DESCRIPTION OF THE DRAWINGS

While the specification concludes with claims particularly pointing outand distinctly claiming the subject matter that is regarded as thepresent invention, it is believed that the invention will be betterunderstood from the following description, which is taken in conjunctionwith the accompanying drawings:

FIG. 1 is a schematic top view of a microvalve employing a mechanicallatch design in accordance with the present invention;

FIG. 2 is an enlarged, perspective view of the microvalve shutterdepicted in FIG. 1;

FIG. 3 is an enlarged, perspective view of a first type of comb driveattached to the shutter as depicted in FIG. 1;

FIG. 4 is an enlarged, perspective view of a second type of comb drivein proximate location to the shutter as depicted in FIG. 1;

FIG. 5 is a top view of the layout for a microvalve in accordance withthe present invention employing a friction latch design;

FIG. 6 is an exploded, schematic cross-sectional view of a pair ofwafers used in accordance with a process to fabricate the microvalve ofthe present invention;

FIG. 7 is a schematic cross-sectional view of the wafers depicted inFIG. 6 which have been bonded together;

FIG. 8 is a schematic cross-sectional view of the wafers depicted inFIG. 7 after slotted openings are etched in the top and bottom wafers, aflow passage substantially aligned therewith is etched in the secondwafer, and the top wafer has been thinned and polished;

FIG. 9 is a schematic cross-sectional view of the wafers depicted inFIG. 8 after the sacrificial layer is removed;

FIG. 10 is a schematic cross-sectional view of the wafers depicted inFIG. 9 when the microvalve is in the closed position;

FIG. 11 a schematic cross-sectional view of the wafers depicted in FIG.9 when the microvalve is in the open position;

FIG. 12 is a schematic top view of the microvalve depicted in FIG. 1when in the latched, closed position;

FIG. 13 is a schematic top view of the microvalve depicted in FIG. 1when in the unlatched, closed position;

FIG. 14 is a schematic top view of the microvalve depicted in FIG. 1when in the unlatched, open position;

FIG. 15 is a schematic top view of the microvalve depicted in FIG. 1when in the latched, open position;

FIG. 16 is an enlarged, top view of a second embodiment for themicrovalve of the present invention in the closed position;

FIG. 17 is an enlarged, top view of the second microvalve embodimentdepicted in FIG. 16 in the open position; and

FIG. 18 is a schematic cross-sectional view of a metal-air batteryincluding at least one microvalve of the present invention to controlflow of air to the cells therein.

FIG. 19 is top plan view of an exemplary battery of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

A preferred embodiment of the present invention involves anelectrostatically-driven MEMS microvalve designed to control fluid flow.In this application, the term “electrostatically-driven” refers to adriving mechanism created from fixed charge due to an electrostaticpotential between two surfaces. This differs from a “thermally-driven”microvalve in that the thermally-driven microvalve utilizes a resistiveelement that provides the heat energy necessary to drive the valve. Sucha resistive element either provides a parasitic drain on the cell itselfor requires an alternative power source to drive the valves. Magnetic orinductive systems, by contrast, use continuous current in a loop togenerate an external magnetic field which in turn creates a magneticforce. An electrostatic valve, however, utilizes the charge of the cellto drive the valve so that the parasitic drain on the cell is much lessthan for thermal or magnetic valves.

In the preferred embodiments, the microvalve is designed to consumepower only during transients, i.e., while changing states from open toclosed or vice versa. More specifically, it will be seen in FIG. 1 thata microvalve, denoted generally by reference number 10, preferablyincludes a shutter 12 located adjacent to and substantially parallelwith a body portion 14 (see FIGS. 6-11), where shutter 12 and bodyportion 14 each preferably have a plurality of spaced openings 16 and18, respectively, formed therein. It will further be seen thatmicrovalve 10 includes a drive mechanism, preferably in the form ofelectrostatic comb drives 20 and 22, to move shutter 12 laterally withrespect to body portion 14 so that spaced openings 16 of shutter 12 arebrought into and out of alignment with spaced openings 18 of bodyportion 14, wherein microvalve 10 is in an open position and a closedposition, respectively. It will be appreciated that the lateral movementof shutter 12 is preferably linear, as indicated by arrows 65 in FIGS.13 and 14. As discussed in greater detail herein, lateral movement ofthe shutter with respect to a body portion may be non-linear (i.e.,rotational) depending upon the configuration of the shutter, the slottedopenings in the shutter and the body portion, and the drive mechanism.Further, the drive mechanism may alternatively comprise a thermal,magnetic or piezoelectric driving mechanism as is known in the art.

Shutter 12 preferably has a substantially rectangular frame 24 havingsides 26 and 28 substantially parallel to spaced openings 16 therein andsides 30 and 32 substantially perpendicular to spaced openings 16, asbest seen in FIG. 2. Electrostatic comb drives 20 and 22 are affixed toshutter frame sides 26 and 28, respectively. Shutter 12 further includesa plurality of spaced finger-like members 34 extending between framesides 30 and 32 to preferably define elongated slots as spaced openings16 therebetween. It will be noted that members 34 are of a linearconfiguration so as to provide linear slots. In this way, a large valveopening may be obtained by moving shutter 12 a relatively shortdistance. Such a design minimizes the power necessary to drivemicrovalve 10 by minimizing the distance shutter 12 needs to bedisplaced. This, in turn, allows the use of electrostatic drivingtechnologies such as MEMS when the power required to drive microvalve 10is lowered to a level that may be practically delivered by thesetechnologies.

It will be understood from FIG. 3 that electrostatic comb drives 20 and22 of the drive mechanism interface with shutter 12 so as to keepshutter 12 suspended above body portion 14. Each electrostatic combdrive 20 and 22 further includes (as shown for electrostatic comb drive20 only) a plurality of suspended ground fingers 21 and a plurality ofanchored fingers 23, wherein ground fingers 21 are pulled to anchoredfingers 23 when a potential is applied therebetween to create anelectrostatic force. Further, a set of resilient beams 25 are providedat an anchored end 27 of each electrostatic comb drive 20 and 22 so asto suspend shutter 12 at frame sides 26 and 28. In this way, shutter 12is able to move between the open and closed positions absent anyfriction forces thereon. In this application, the term “resilient beams”refer to mechanical structures that undergo displacement so as toprovide a spring-like restoring force on the whole system.

Microvalve 10 also includes a latching mechanism for preventing shutter12 from moving laterally with respect to body portion 14. In oneembodiment, the latching mechanism preferably includes ears 36 and 38extending from frame sides 30 and 32, respectively, and electrostaticcomb drives 40 and 42 positioned adjacent frame sides 30 and 32 whichare movable so as to engage and disengage ears 36 and 38 and therebymechanically prevent and permit shutter 12 from moving, respectively. Ofcourse, only one ear and corresponding electrostatic comb drive may berequired to prevent lateral movement of shutter 12. It will beappreciated from FIG. 4 that electrostatic comb drives 40 and 42likewise include (as shown for electrostatic comb drive 40 only) aplurality of suspended ground fingers 41 and a plurality of anchoredfingers 43, wherein ground fingers 41 are pulled to anchored fingers 43when a potential is applied therebetween to create an electrostaticforce. In this way, a beam 49 is brought into and out of engagement withears 36 and 38. A set of resilient beams 45 are also provided at ananchored end 47 of each electrostatic comb drive 40 and 42.

An alternative embodiment of the latching mechanism is shown in FIG. 5,where a shutter 50 similar to that described hereinabove forms the spinefor a plurality of electrostatic comb drives 52. Accordingly, shutter 50is moved laterally (in a substantially linear fashion) with respect to abody portion as ground fingers 54 are pulled to anchored fingers 56 inelectrostatic comb drives 52 when a potential is applied therebetween.It will be seen that the latching mechanism for this embodimentpreferably includes at least one electrostatic comb drive 58 positionedsubstantially perpendicular to shutter 50 so that it is able to engageand disengage the frame thereof and thereby frictionally prevent andpermit shutter 50 from moving, respectively.

In operation, it will be appreciated from FIGS. 12-15 that the method ofelectrostatically actuating microvalve 10 between a first (closed)position and a second (open) position preferably includes the steps ofdisengaging the latching mechanism (as indicated by arrows 55 in FIG. 12and the relative positioning of the latching mechanism in FIG. 13) so asto permit movement of shutter 12 with respect to body portion 14,actuating a drive mechanism to move shutter 12 from the first positionto the second position with respect to body portion 14 (as indicated byarrows 65 in FIG. 13 and the open position designation for shutter 12 inFIG. 14), and engaging the latching mechanism so as to prevent movementof shutter 12 from the second position (as indicated by arrows 67 inFIG. 14 and the relative positioning of the latching mechanism in FIG.15). Further, the disengaging step preferably occurs immediately priorto and during movement of shutter 12 and the actuating step occurs onlywhile the latching mechanism is disengaged.

In a preferred embodiment, shutter 12 is biased in a closed position sothat the latching mechanism is utilized to prevent shutter 12 frommoving when in the open position. Alternatively, shutter 12 may bebiased in an open position so that the latching mechanism is utilized toprevent shutter 12 from moving when in the closed position. In this way,power to electrostatic comb drives 20 and 22 of the drive mechanism willneed to be maintained only while moving shutter 12 in the unbiaseddirection. Of course, power to electrostatic comb drives 20 and 22 ofthe drive mechanism is maintained only while the latching mechanism isdisengaged whether biasing of shutter 12 occurs or not.

Similarly, the latching mechanism is preferably biased in a closed orlocked position. This has the desirable effect of requiring power onlyto disengage electrostatic comb drives 40 and 42. Consequently, it willbe understood that power to the latching mechanism is preferablymaintained only during a change in position of shutter 12 (see FIGS. 13and 14).

In conjunction with a method of fabricating microvalve 10 describedhereinafter, it will be appreciated from FIGS. 6-11 that shutter 12,electrostatic comb drives 20 and 22 of the drive mechanism, andelectrostatic comb drives 40 and 42 of the latching mechanism are etchedon a masking material 59 (preferably made of oxide) located at a topsurface 62 of a first wafer 60. Body portion 14 is a second wafer 64which has spaced openings 18 etched on a top surface 66 thereof, whereina portion of second wafer 64 in substantial alignment with spacedopenings 18 is etched therefrom so as to create a passage 68 in flowcommunication therewith (see FIG. 8). First and second wafers 60 and 64are preferably constructed of a single crystal silicon. A sacrificiallayer 70 is positioned between first and second wafers 60 and 64 to bondthem together. It will be appreciated that sacrificial layer 70,preferably in the form of an oxide, may be attached to a bottom surface63 of first wafer 60, top surface 66 of second wafer 64, or both. Aportion of sacrificial layer 70 is removed in substantial alignment withflow passage 68 (see FIG. 9) so as to permit fluid flow (as depicted byarrows 61) through microvalve 10 when spaced openings 16 in first wafer60 are aligned with spaced openings 18 in second wafer 64 (see FIG. 11).Otherwise, fluid flow through microvalve 10 is substantially preventedwhen spaced openings 16 and 18 are in substantial misalignment (see FIG.10). It will be appreciated that a thickness t of sacrificial layer 70is predetermined so that a designated leakage flow (as depicted bydashed arrows 69 in FIG. 10) through microvalve 10 is permitted whenspaced openings 16 and 18 are in the closed position. Of course, shutter12 may be moved to a position intermediate the open and closed positions(i.e., partial alignment of spaced openings 16 and 18) and mechanicallyor frictionally latched or held in place to permit partial opening ofmicrovalve 10 and a proportional amount of fluid flow therethrough. Inthe mechanical latching design, this will typically entail either ears36 and 38 being located asymmetrically along shutter frame 24 orproviding additional ears on at least one side thereof.

It will be understood, then, that a method of fabricating anelectrostatic microvalve like that described herein preferably involvesthe following steps: providing a first wafer 60 having top surface 62and a bottom surface 63; providing a masking material 59 on top surface62 of first wafer 60; providing a second wafer 64 having a top surface66; etching a plurality of spaced openings 18 on a top surface 66 ofsecond wafer 64; bonding a bottom surface 63 of first wafer 60 to topsurface 66 of second wafer via a sacrificial layer 70; etching maskingmaterial 59 of first wafer 60 to create a shutter 12 and a drivingmechanism (e.g., a plurality of electrostatic comb drives 20, 22, 40 and42) operative therewith; etching a portion of second wafer 64 so as tocreate a passage 68 in flow communication with spaced openings 18 etchedon top surface 66 thereof; and, removing a portion of sacrificial layer70 between first and second wafers 60 and 64 to release the drivingmechanism, as well as provide flow communication between spaced openings16 and 18 of first and second wafers 60 and 64, respectively. It will berecognized from a comparison of FIGS. 6 and 7 that steps of thinning andpolishing first wafer 60 to a predetermined thickness prior to theapplication of masking material 59 on first wafer 60 is preferred.

A second embodiment of the microvalve of the present invention,indicated generally by reference numeral 200, is depicted in FIG. 16. Asseen therein, microvalve 200 is of the type where the lateral movementbetween a shutter 212 and a body portion 214 positioned in proximatelocation is non-linear (i.e., rotational). In particular, shutter 212preferably has a substantially circular frame 224 with a center portion215, although other symmetrical shapes may be utilized. It will be seenthat shutter 212 has a plurality of spaced members 234 extending betweenframe 224 and center portion 215, with openings 216 providedtherebetween. Members 234 are shown as having a curved configurationwith openings 218 in body portion 214 being arranged so as to align withmembers 234 when microvalve 200 is in a closed position (preventingfluid flow) and to align with openings 216 when microvalve 200 is in anopen position (permitting fluid flow). For their part, members 234 mayhave any desired shape (e.g., a straight spoke) in addition to thatshown. Likewise, openings 218 in body portion 214 may involve openingshaving any desired shape, including small openings spaced closelytogether, slots, or any other design, providing they conform to theshape and arrangement of members 234 and openings 216 of shutter 212 (asshown in FIGS. 16 and 17).

With respect to the drive mechanism for microvalve 200, first and secondcomb drives 220 and 222 are connected to shutter 212 at oppositelocations along frame 224. Since lateral movement of shutter 212 isintended to be non-linear, comb drives 220 and 222 each have suspendedground fingers 221 and anchored fingers 223 which are arcuate in design.In this way, ground fingers 221 are pulled to anchored fingers 223 whena potential is applied therebetween to create an electrostatic force andshutter 212 is caused to rotate. Although comb drives 220 and 222 areshown as causing shutter 212 to rotate counterclockwise, they may bealtered to cause a clockwise rotation. A set of resilient beams 225 areprovided at an anchored end 227 of each electrostatic comb drive 220 and222 so as to suspend shutter 212 at frame 224. Accordingly, shutter 212is able to move between the open and closed positions absent anyfriction forces thereon.

Microvalve 200 also includes a latching mechanism for preventing shutter212 from moving laterally with respect to body portion 214. In oneembodiment, the latching mechanism preferably includes at least one ear236 extending from frame 224 and an electrostatic comb drive 240positioned adjacent frame 224 which is movable so as to engage anddisengage ear 236 and thereby mechanically prevent and permit shutter212 from moving, respectively. Of course, additional ears andcorresponding electrostatic comb drives may be utilized to furtherenhance the performance of the latching mechanism. It will beappreciated that electrostatic comb drive 240 is similar toelectrostatic comb drive 40 described hereinabove, where it includes aplurality of suspended ground fingers 241 and a plurality of anchoredfingers 243 so that ground fingers 241 are pulled to anchored fingers243 when a potential is applied therebetween to create an electrostaticforce. In this way, a beam 249 is brought into and out of engagementwith ear 236. A set of resilient beams 245 are also provided at ananchored end 247 of electrostatic comb drive 240. Instead of engagingear 236, the latching mechanism may involve beam 249 of electrostaticcomb drive 240 merely providing a substantially perpendicular frictionalforce by engaging frame 224.

It will be understood that microvalve 200 will preferably be operatedand fabricated like microvalve 10 as discussed in detail above, with theonly changes being in the non-linear type of lateral movement betweenshutter 212 and body portion 214 and modifications to the actuatorsnecessitated thereby. Likewise, shutter 212 may be biased in either theclosed or open positions so that power to electrostatic comb drives 220and 222 is preferably maintained only during movement of shutter 212 inone direction. Of course, this will occur only when the latchingmechanism is disengaged. Likewise, the latching mechanism is preferablybiased in a closed position so that power to electrostatic comb drive240 is required only to disengage it. Clearly, then, power to thelatching mechanism is maintained only during a change in position ofshutter 212.

One aspect of the present invention is directed to anelectrostatically-driven MEMS microvalve that may be used to controlfluid (gas or liquid) flow into and/or out of a battery, a batteryincluding such a valve, or a method of controlling fluid flow intoand/or out of a battery. The battery may include, for example, one ormore metal-air cells, one or more fuel cells, one or more voltaic cells,or a combination of these to produce a hybrid cell. In each case, thefluid flow enables or assists the provision of electrical power byproviding a fluid cathode such as in the case of a metal-air cell, byproviding a fluid anode in the case of a fuel cell, or by providing afluid electrolyte such as in the case of a voltaic cell used inseawater.

FIGS. 18 and 19 show a cross-section of an exemplary fluid-breathingvoltaic battery 75 having a container 76 and at least one voltaic cell78 disposed within container 76. Container 76 may have a cylindricalshape as shown, a prismatic shape, or even a flat round shape (i.e., abutton cell). A fluid exchange system for battery 75 includes at leastone microvalve 100 of the present invention and a controller 104electrically connected thereto to control the flow of fluid in battery75. It will be understood that controller 104 is preferably like thatdescribed in a U.S. Pat. No. 6,074,775, entitled “Battery Having aBuilt-in Controller” and issued on Jun. 13, 2000, which is herebyincorporated by reference. Microvalve 100 may be located adjacent a topportion of an air path 82 in battery 75. Microvalve 100 is retained inposition by a valve seat 87 (which also preferably includes a portionfor crimping a top seal 114) and preferably has a hydrophobic layer 88(e.g., polytetrafluoroethylene or polypropylene) located between it andopenings 84 in a top metal cover 86 to diffuse air entering path 82. Aplurality of openings 84 are preferably spaced circumferentially in topmetal cover 86, in such quantities and size as needed for a desired airflow into battery 75.

A second microvalve 102 may be located adjacent a bottom portion of airpath 82 so as to control air flow entering from openings 92 in a bottommetal cover 94. Microvalve 102 is likewise retained in position by avalve seat 96 (which, like valve seat 87, preferably includes a portionfor crimping a bottom seal 115) and preferably has a hydrophobic layer98 located between it and openings 92 to diffuse air entering path 82.While hydrophobic layers 88 and 98 are shown as being located on onlyone side of microvalves 100 and 102, several additional or alternativelocations are also possible. For example, hydrophobic layers could beplaced on both sides of each microvalve 100 and 102 in order to limitthe flow of water vapor into or through each microvalve. Additionally,hydrophobic layers could be placed in substantial alignment withopenings 84 and 92 in top and bottom metal covers 86 and 94,respectively. It will also be understood that materials for removingcarbon dioxide could be incorporated in the same positions as thehydrophobic membranes.

Controller 104 is preferably positioned at the negative end of the cellsince both positive and negative battery connections are readilyaccessible at this location. While controller 104 is preferablyelectrically connected to both microvalve 100 and microvalve 102 (andany other microvalves in battery 75), a separate controller for eachmicrovalve may be utilized. A controller located at the positive end ofthe cell, however, would require a line to be run from the negative endof the cell to provide a negative connection. Several other alternativelocations are possible for controller 104, including the inner surfaceof top or bottom metal covers 86 and 94, on top of valve seats 87 and96, or even incorporated in microvalves 100 and 102 themselves.

It will be understood that connections are necessary between thepositive and negative terminals of battery 75, microvalves 100 and 102,and controller 104. Of course, valve seats 87 and 96 for microvalves 100and 102, respectively, are preferably metal assemblies which carry thepositive battery charge. A wire connection 85 is preferably providedbetween top metal cover 86 and valve seat 87, because lowering top metalcover 86 and spot welding it to valve seat 87 could inhibit air flowfrom openings 84 to air path 82 unless additional measures were taken(i.e., if openings in top metal cover 86 were located in a middle regionabove hydrophobic layer 88 or if top metal cover 86 was constructed froma metal screen, perforated metal, or expanded metal). Wire connections89, 91 and 93 are then provided between the negative terminal forbattery 75 and controller 104, between controller 104 and microvalve102, and between microvalve 102 and microvalve 100, respectively.

It will be appreciated that additional microvalves, preferably in theform of an array, may be positioned within battery 75 as an alternativemanner of controlling the amount of air entering therein. In this way,the amount of airflow (dependent upon the number of microvalves open)permitted to flow therein is able to provide a high current rate withoutcontinued exposure to ambient air after the load has been removed. Sincethe microvalves for such an array can be of a bi-stable design (i.e.,open or closed), this is an attractive alternative to having microvalve100 and/or microvalve 102 be only partially open. Although not shown,one or more microvalves for battery 75 may be located adjacent aperiphery of container 76.

The terms “electrically connected” and “electrical connection” refer toconnections that allow for continuous current flow. The terms“electronically connected” and “electronic connection” refer toconnections in which an electronic device such as a transistor or adiode are included in the current path. “Electronic connections” areconsidered in this application to be a subset of “electricalconnections” such that while every “electronic connection” is consideredto be an “electrical connection,” not every “electrical connection” isconsidered to be an “electronic connection.”

It will further be seen that voltaic cell 78 of battery 75 preferablyincludes an air cathode 108, a metal anode 110, and a separator 112therebetween. Seals 114 and 115 of an insulating material are providedat each end of voltaic cell 78, with top valve seat 87 being in contactwith air cathode 108. Another hydrophobic layer may be located betweenair path 82 and air cathode 108 if necessary. Of course, other batteryconfigurations may employ the microvalves described herein, includingone where the anode is a cylindrical plug in the center of the cellsurrounded by an air cathode on the outside. Another alternative designinvolves the anode and air cathode being configured in a spiral or“jelly roll” configuration. It will be understood that othermodifications may be required in order to employ these alternativebattery designs, such as including an air channel between the containerand the air cathode and having openings formed in a side portion of thecase instead of the ends.

Controller 104 individually, or in conjunction with a second controller,is preferably utilized to open and/or close microvalves 100 and 102. Theterm “controller” as used in this application refers to a circuit thataccepts at least one input signal and provides at least one outputsignal that is a function of the input signal. Controller 104 maymonitor and/or manage fluid flow between a metal-air electrochemicalcell and the external environment. For example, controller 104 may allowair into voltaic cell 78 when oxygen is required to provide the currentrequired by the load. When the load is disconnected or demands only aminimal amount of current, controller 104 may close or partially closemicrovalves 100 and 102 so that the reaction in voltaic cell 78 isstopped or slowed down and the cell electrolyte is protected until theload demands more current. At that time, controller 104 may openmicrovalve 100 so that voltaic cell 78 will generate the currentdemanded by the load. In this regard, it will also be appreciated thatvoltaic cell 78 preferably provides power to microvalves 100 and 102 andis able to do so due to the leakage flow therethrough even when in theclosed position. Optimally, controller 104 and/or a second controllerwill provide signal conditioning to the power provided by voltaic cell78 to drive microvalves 100 and 102.

Controller 104 may also be used to perform other functions to furtherincrease the operation efficiency and/or safety of one or moreelectrochemical cells in addition to controlling fluid flow into and/orout of one or more electrochemical cells. Examples of operations thatmay be performed by controller 104 include: using a DC/DC converter toextend the service run time of the battery; controlling a charge cycleof the electrochemical cell by directly monitoring the electrochemicalproperties of that particular cell; providing a safety disconnect in theevent of overheating, inverse polarity, short-circuit, over-pressure,overcharge, over-discharge or excessive hydrogen generation; and,monitoring the state of charge of that particular electrochemical cellto provide this information to the user, the device, or for qualityassurance purposes. Functions such as these are described in detail inU.S. Pat. Nos. 6,074,775 and 6,163,131, each entitled “Battery Having aBuilt-in Controller”, which are both incorporated by reference in thisapplication.

While particular embodiments and/or individual features of the presentinvention have been illustrated and described, it would be obvious tothose skilled in the art that various other changes and modificationscan be made without departing from the spirit and scope of theinvention. Further, it should be apparent that all combinations of suchembodiments and features are possible and can result in preferredexecutions of the invention.

1. A fluid-breathing voltaic battery, comprising: (a) a container; (b) avoltaic cell disposed within said container; and (c) a fluid exchangesystem comprising: (1) a microvalve having a first state and a secondstate, said microvalve being disposed in said container such that saidmicrovalve as located between a fluid flow and said cell, wherein saidmicrovalve is adapted to allow a fluid into said cell when saidmicrovalve is in said first state and to substantially prevent saidfluid from flowing into said cell when said microvalve is in said secondstate; (2) a controller electrically connected to said microvalve, saidcontroller being adapted to initiate a change of state in saidmicrovalve; and (3) a latching mechanism for retaining said microvalvein one of said first and second states. (d) wherein said microvalvecomprises: (1) a body portion having a plurality of spaced openingsformed therein; (2) a wafer located adjacent and substantially parallelwith said body portion, said wafer comprising a shutter integralthereto, said shutter having a plurality of spaced openings formedtherein; (3) said wafer further comprising a drive mechanism integralthereto for causing said shutter to move with respect to said bodyportion so that said spaced openings of said shutter are brought intoand out of alignment with said spaced openings of said body portion,said drive mechanism being electrically connected to said controller;and, wherein said wafer further comprises said latching mechanism. 2.The fluid-breathing voltaic battery of claim 1, wherein said first stateis an open position and said second state is a closed position.
 3. Thefluid-breathing voltaic battery of claim 2, wherein said shutter isbiased in said closed position.
 4. The fluid-breathing voltaic batteryof claim 3, wherein said latching mechanism is utilized to prevent saidshutter from moving when in said open position.
 5. The fluid-breathingvoltaic battery of claim 2, wherein said shutter is biased in said openposition.
 6. The fluid-breathing voltaic battery of claim 5, whereinsaid latching mechanism is utilized to prevent said shutter from movingwhen in said closed position.
 7. The fluid-breathing voltaic battery ofclaim 1, wherein said shutter further comprises: (a) a substantiallycircular frame having a center portion; and, (b) a plurality of spacedmembers extending between said center portion and said frame definingsaid spaced openings therebetween.
 8. The fluid-breathing voltaicbattery of claim 7, said latching mechanism further comprising: (a) anear extending from said frame; and, (b) an electrostatic comb driveposition adjacent said frame proximate said ear, wherein saidelectrostatic comb drive is movable so as to engage and disengage saidear and thereby prevent and permit said shutter from moving,respectively.
 9. The fluid-breathing voltaic battery of claim 7, saidlatching mechanism further comprising at least one electrostatic combdrive position adjacent said frame, wherein said electrostatic combdrive is movable so as to engage and disengage said shutter frame andthereby prevent and permit said shutter from moving, respectively. 10.The fluid-breathing voltaic battery of claim 7, wherein said drivemechanism is an electrostatic comb drive attached to said frame.
 11. Thefluid-breathing voltaic battery of claim 1, wherein power to disengagesaid latching mechanism is maintained only during a change in positionof said shutter.
 12. The fluid-breathing voltaic battery of claim 1,wherein power to said drive mechanism is maintained only while saidlatching mechanism is disengaged.
 13. The fluid-breathing voltaicbattery of claim 1, wherein said shutter is movable to a positionintermediate said open position and said closed position so as to permita partial opening of said microvalve.
 14. The fluid-breathing voltaicbattery of claim 1, wherein said drive mechanism causes said shutter tomove linearly with respect to said body portion and said latchingmechanism prevents said shutter from moving with respect to bodyportion.
 15. The fluid-breathing voltaic battery of claim 1, whereinsaid drive mechanism causes said shutter to move non-linearly withrespect to said body portion and said latching mechanism prevents saidshutter from moving with respect to said body portion.
 16. Thefluid-breathing voltaic battery of claim 1, wherein a predeterminedamount of fluid is able to leak through said microvalve in said secondstate.